1. Field of the Invention
The present invention generally relates to RF receive chains and, more specifically, to chains provided with a so-called low-noise amplifier (LNA) associated with a mixer to convert an RF signal at a relatively high frequency into a signal at a relatively low intermediary frequency with respect to the frequency of the received signal, to recover the envelope of the RF modulation.
The present invention more specifically applies to mobile telephony receive chains.
2. Discussion of the Related Art
FIG. 1 is a block diagram of a conventional example of an RF chain. AN RF signal received by an antenna 1 is processed by a low-noise amplifier 2 (LNA), then by a mixer 3 receiving an intermediary frequency and switching at a so-called local oscillator frequency OL. The output of mixer 3 is intended to be processed by receive circuits 4 (Rx). Most often, filters 5 and 6 respectively at the frequency of the received signal and at the intermediary frequency are provided upstream and downstream of the series-association of amplifier 2 and of mixer 3. Further, other circuits may be interposed between antenna 1 and filter 5. These circuits have been symbolized by an interruption 8 in the path of the signal and may comprise path separators, couplers, impedance matching circuits, etc.
To improve the stability and the frequency matching, as well as the linearity of the reception chain, a reverse feedback 21 is generally performed on low-noise amplifier 2 by feeding back on its input a portion of its output signal. This reverse feedback loop 21 may comprise a filter 22 at the frequency of the RF signal. Such a reverse feedback enables doing away with the use of inductive elements used in impedance-matching circuits, but adversely affects the gain as well as the signal-to-noise ratio of the chain.
Document WO-A-2007/057324 describes an example of an RF reception system in which a signal sampled from the output of a mixer is applied to a low-noise amplifier. This amounts to permanently applying a reverse feedback signal.
Document US-A-2006/0208791 describes another example of an RF receiver using a switched-capacitor filter.